I bought myself a server with 40TB of storage and then gave 10 of that to a iscsi drive which I mount from my gaming pc. So I now install games over my network and play them from there. It helps when you have over 150 steam games and want to install them all.
So if the rare occasion appears when I actually play one of them I can do so instantly.
I had the same problem. My SSD is constantly full so I bought a cheap external HDD and instead of deleting&redownloading games I just move them between disks.
I just have 2TB ssd space. Not realy fast ones, so it wasn't that expensive, but they do the trick. Usually when I am deleting something there allways a game that I haven't played in a long time.
Oh yes thats painfull especialy with slow internet. Its not that much of a problem if the game has a few gb but when its somethig like tw Warhammer that has a god damm 100gb ,you need to be realy realy sure you wont get a sudden urge to play it next week.